For the purposes of this section, miles traveled under district ownership by all buses owned by a district on the date of application for replacement allowance are divided as follows:
The replacement allowance approved for a school bus shall be reduced if the total miles traveled by all of the buses for the purpose of subsection (a)(4) exceed 20 percent of the total miles traveled by all of the buses for the purposes of subsections (a)(1) and (a)(4).
The reduction shall be in an amount which is the same percentage of the approved replacement expense as the excess mileage is of the total miles traveled under district ownership by all of the buses for the purposes of subsections (a)(1) and (a)(4).
Cal. Code Regs. Tit. 5, § 15325